1. Field of the Invention
The present invention relates to prostheses for implantation into a mammalian vessel, and in particular, to intraluminal stents that are provided with a cover that can deliver and release drugs.
2. Description of the Prior Art
The treatment of stenosis is the subject of much research and discussion. Stenosis are currently being treated by a number of well-known procedures, including balloon dilatation, stenting, ablation, atherectomy or laser treatment.
Restenosis is the renarrowing of a peripheral or coronary artery after trauma to that artery caused by efforts to open a stenosed portion of the artery, such as by balloon dilatation, ablation, atherectomy or laser treatment of the artery. For such procedures, restenosis occurs at a rate of about 20-50% depending on the definition, vessel location, lesion length and a number of other morphological and clinical variables. Restenosis is believed to be a natural healing reaction to the injury of the arterial wall that is caused by angioplasty procedures. The host reaction begins with the thrombotic mechanism at the site of the injury. The final result of the complex steps of the healing process can be intimal hyperplasia, the uncontrolled migration and proliferation of medial smooth muscle cells, combined with their extracellular matrix production, until the artery is again stenosed or occluded.
Many attempts have been made or suggested to treat stenosis, and to prevent or minimize restenosis. One common approach is to implant intravascular stents in coronary and peripheral vessels. The stent is usually inserted by a delivery system (e.g., such as a catheter) into a vascular lumen and expanded (either via a balloon on a catheter, or through self-expansion) into contact with the diseased portion of the arterial wall to provide mechanical support for the lumen. The positioning of the stent in the lumen can be used to treat stenosis by re-opening the lumen that had been partially blocked by the stenosis. However, it has been found that restenosis can still occur with such stents in place. In addition, a stent itself can cause undesirable local thrombosis. To address the problem of thrombosis, persons receiving stents also receive extensive systemic treatment with anti-coagulant and antiplatelet drugs.
To address the restenosis problem, a number of approaches have been suggested. One type of approach relates to the delivery of drugs to minimize restenosis. As one example, these drugs can be delivered via oral, intravascular or intramuscular introduction, but these attempts have been largely unsuccessful. Unfortunately, pills and injections are known to be ineffective modes of administration because constant drug delivery and higher local concentration are very difficult to achieve via these means. Through repeated doses, these drugs often cycle through concentration peaks and valleys, resulting in time periods of toxicity and ineffectiveness.
Localized drug delivery is another example. There were many different attempts to provide localized drug delivery. One example of localized drug delivery is to provide the metallic walls or wires of the stents with therapeutic substances, fibrin and other drugs that can be released over a period of time at the diseased location of the vessel. However, the incorporation of drugs into the walls or wires of the stent may significantly compromise the strength of the stent.
A second example of localized drug delivery is to incorporate a drug into a stent that is constructed not of metal but of a biodegradable polymer. However, the loading in and releasing of drugs from a polymeric stent may change the structural integrity and mechanical properties of the stent.
A third example of localized drug delivery is to directly coat the metal stent with a polymer that is bonded to or contains the desired drugs or anti-stenotic substances. Unfortunately, such polymer-coated stents have not been completely effective in preventing restenosis because of the cracking of the polymer as the stent is being expanded during deployment, saturation of the drug binding sites on the stent, and other reasons.
A fourth example of localized drug delivery is to provide a polymer sleeve or sheath that encompasses a portion of the stent. The sleeve or sheath would operate as a local drug delivery device. In some instances, the sheath or sleeve is made up of a bioabsorbable polymer that incorporates a drug, with the sheath or sleeve having a thickness to allow for controlled release of the drug. However, this approach suffers from the drawback that very few drugs are capable of being incorporated with common solid state polymers. In addition, directional release of drug to either the lumen or the arterial wall cannot be achieved. It will also be problematic for medical practitioners to select the type of drug and the dosage of the drug to be used, as well as the stent type to be implanted.
In addition to the problems of stenosis and restenosis, the development of cancerous blockages inside body passageways (e.g., esophagus, bile ducts, trachea, intestine, vasculature and urethra, among others) can also be treated with stents, which operate to hold open passageways which have been blocked by the cancerous growth or tumors. However, the stents do not prevent the ingrowth of the cancerous material through the interstices of the stent. If the ingrowth reaches the inside of the stent, it might result in blockage of the body passageway in which the stent had been implanted.
In addition to the above-described problems experienced by localized drug delivery, conventional stents are also ineffective in preventing the ingrowth of host tissue proliferation or inflammatory material through the interstices of the stent. Some inflammatory reactions may be associated with vulnerable plaque or other unknown causes.
Traditional scientific wisdom holds that heart attacks originate from severe blockages created by atherosclerosis (i.e., the progressive build-up of plaque in the coronary arteries). The increase of lipids in the artery and the ensuing tissue reaction lead to narrowing of the affected vessel which, in turn, can result in angina and eventual coronary occlusion, sudden cardiac death, and thrombotic stroke. However, research conducted in the past decade is leading to a shift in understanding of atherosclerosis and pointing the way to major changes in the diagnosis and treatment of some kinds of life threatening forms of heart disease.
Scientists theorize that at least some coronary diseases are inflammatory processes, in which inflammation causes plaque to rupture. These so-called xe2x80x9cvulnerable plaquesxe2x80x9d do not block the arteries. On the other hand, much like an abscess, they are ingrained under the arterial wall, so that they are undetectable. They cannot be seen by conventional angiography or fluoroscopy, but they cause symptoms such as shortness of breath or pain. Yet, for a variety of reasons, they are more likely to erode or rupture, creating a raw tissue surface that forms scabs. Thus, they are more dangerous than other plaques that cause pain, and may be responsible for as much as 60-80% of all heart attacks.
As used herein, the term xe2x80x9crestenosisxe2x80x9d is defined to be a natural healing reaction to the injury of the arterial wall that is caused by angioplasty procedures. xe2x80x9cRestenosisxe2x80x9d is not associated with vulnerable plaque. The host reaction begins with the thrombotic mechanism at the site of the injury. The final result of the complex steps of the healing process can be intimal hyperplasia, which is the uncontrolled migration and proliferation of medial smooth muscle cells, combined with their extracellular matrix production, until the artery is again stenosed or occluded as typically observed in a stable plaque as opposed to a vulnerable plaque.
Thus, there still remains a need for a prosthesis that provides effective localized drug delivery to minimize or prevent restenosis and the ingrowth of host tissue proliferation or inflammatory material through the interstices of the stent, while avoiding the disadvantages set forth above. In addition, there remains a clinical need for a method for treating vulnerable plaque.
It is an object of the present invention to provide an intraluminal prosthesis that minimizes or prevents the ingrowth of host tissue proliferation or inflammatory material through the interstices or ends of a stent.
It is another object of the present invention to provide an intraluminal prosthesis that provides effective localized drug delivery.
It is yet another object of the present invention to provide an intraluminal prosthesis that provides site-specific drug delivery and/or evenly distributed drug delivery for treating a region of the intraluminal surface.
It is yet a further object of the present invention to provide a method for treating vulnerable plaque.
In order to accomplish the objects of the present invention, there is provided a prosthesis having a stent, and a cover that covers a portion of the stent. The cover has at least two layers of materials. In one embodiment of the present invention, the layers of material define at least one chamber therebetween, and a drug is loaded into the chamber by a drug dispersing element. In another embodiment of the present invention, the drug can be loaded into the material of one or more layers. The cover can be provided inside the luminal walls of the stent, or the stent can be retained in the at least one chamber. The cover of the present invention can be deployed to treat vulnerable plaque.